public class Demo2 {
    private static int count = 0;
    private void add(){
        synchronized (this){
            count++;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Demo2 d = new Demo2();
        Thread t1 = new Thread(() -> {
            for(int i = 0; i < 50000; i++){
                synchronized (d){
                    d.add();
                }
            }
        });
        Thread t2 = new Thread(() -> {
            for(int i = 0; i < 50000; i++){
                synchronized (d){
                    d.add();
                }
            }
        });
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(count);
    }
}
